When using etching gases such as chlorine trifluoride (ClF3) or bromine trifluoride (BrF3), after the adsorption of these compounds on a silicon surface, by the liberation of fluorine radicals, which react with silicon atoms of the available silicon surface to form spontaneously volatile silicon fluoride compounds, which subsequently leave the surface, silicon is spontaneously etched. This being the case, silicon removal occurs in those places where silicon and the etching gas used come into contact with each other. In this connection, it is known that the etching rate increases with a falling temperature of the silicon surface and a rising partial pressure of the process gas. It is also known that, using these etching methods, removal rates of more than 10 μm per minute are possible at correspondingly high pressures in the range of a few mbar to a few 10 mbar, a corresponding material quantity supply of the reactive gas of a few 100 sccm (cm3 of gas flow per minute at a pressure of 1 bar) up to a few slm (dm3 gas flow per minute at a pressure of 1 bar) and a low substrate temperature of, for example, −20° C. to +20° C. The etching continues as long as a suitable process gas and silicon are available.
The explained gas phase etching of silicon using a halogen trifluoride has the advantage of extraordinarily high selectivity compared to most non-silicon materials, so that it is very simple to mask this etching. In particular, even thin photo-resist masks, hard material layers of SiO2, silicon nitride or silicon oxynitride are sufficient as masking layers to define the silicon areas to be etched and the silicon areas not be etched. In practice, such masking layers are not measurably removed, so that even very deep or long silicon etchings are possible without the requirement for thick masking layers.
In addition, it is known that, by a directed supply of the etching gas onto a silicon surface, using, for example, a nozzle or other outflow device, silicon is able to be etched in a locally masked or even unmasked manner, such as by moving the nozzle over the silicon surface. This being the case, it is thereby even possible to “write” in silicon or to “cut” silicon.
In the explained spontaneous gas phase etching of silicon, it is of advantage that no plasma at all or similar activation of the reactive gas is necessary. This being the case, this method gains increased importance in the case of sacrificial layer techniques in silicon micromechanics, particularly for manufacturing cavities and diaphragms, e.g., for pressure sensors, hot film sensors, microphones, etc., as well as generally for structures in which the important thing is not so much a lateral dimensional accuracy, and a certain under-etching of edges is tolerable.
Besides the mentioned high selectivity compared to non-silicon, the advantages of gas phase etching are the simplicity of the etching devices used and the very high removal rates at low gas cost.
However, in using such a method for manufacturing micromechanical structures, such as cavities or diaphragms, a drawback is the fact that a simple and effective etching stop is missing. This being the case, up to this point, a cavity to be generated in a silicon wafer was simply etched so deeply that a hole was created in the wafer, or the creation of a diaphragm or cavity having a defined thickness or depth was performed on a purely time-controlled basis, which is difficult to control and inaccurate.